Re: linux-next: manual merge of the security-testing tree with the tree

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 30, 2011, 18:52 UTC
Message-ID: <7v4o37qhi6.fsf@alter.siamese.dyndns.org>
In-Reply-To: <BANLkTi=usrsF60Popfu55v3_dXe7Yo0GrA@mail.gmail.com>

Linus Torvalds <torvalds@linux-foundation.org> writes:

Show 10 quoted lines

> It would be lovely if "git show" (and log operations) had some option
> to do a "expensive merge check" and did actually figure out the common
> ancestor and at least took that into account.
>
> It would be doable to do it at least better than we do now - the
> common ancestor is not cheap to compute, but it's much cheaper than a
> full merge, and would at least allow us to flag dangerous merges. Of
> course, it gets fun when there are multiple common ancestors and
> renames. It's entirely possible that it's never going to be practical
> to do anything but "re-do the merge and compare result".

I would have to say that it would boil down to "re-do the merge" whichever way we implement it, and it is not necessarily a bad thing.

There are ideas to implement a mode of "git merge" that works entirely in-core without touching the working tree (it may have to write temporary blobs and possibly trees to the object store, though). It would let sites like github to let its users accept a trivial pull request that can merge cleanly on site in the browser without necessarily having to have a local checkout used for conflict resolution.

If such an "in-core merge" feature is implemented cleanly in a reusable way, it would be just the matter of comparing the output from it with the actual committed result.

Of course, if the committed result was deliberately made by "-s ours", comparison between an auto-merge result and the committed result would produce a lot of noise, but that is really the point of "expensie merge check", so the noise in that scenario is a feature, not a bug.